Paperboard products having weakened severance lines or lines of weakness which may be readily torn apart are used throughout the paperboard carton industry. To form such severance lines, a cutting tool is generally used having an interrupted cutting edge. When the tool is applied to a sheet of paperboard, a series of short, aligned, spaced apart cuts in the form of perforations passing through the paperboard are formed. The paperboard is subsequently torn along the line of weakness, and in the case of a carton having an access opening in a top panel thereof defined by the line of weakness permits the access opening to be torn out by applying an upward force on the access panel. Although the type of severance line discussed hereinabove is satisfactory for many applications, its use presents numerous problems when certain types of paperboard are used, when it is desired to achieve a clean severance of the access opening or when the line of weakness is of a high degree of curvature.
When paperboard material having a strong outer surface layer which is somewhat stronger than the remainder of the thickness of the paperboard material, that is when a paperboard material having a decorative coating on the outer surface thereof is used, or when some of the fibers of the paperboard material are oriented transversely with respect to the line of perforations, a clean separation at the perforations may not occur, thus resulting in the paperboard peeling in an area outside the access opening defined by the line of weakness. Moreover, when the severance line is of a high degree of curvature, the portion of the paperboard material between the perforations at a high degree of curvature may continue outwardly through the top surface of the carton thereby destroying the decorative outer surface.
One solution to the above-noted problem is to provide a line of weakness formed of a series of perforations passing completely through the paperboard material along with secondary cuts which pass only partially through the surface of the board. This type of severance line has proved to overcome some of the above-noted shortcomings; however, the formation of such line on a mass production basis has proven to be difficult. That is, such a severance line must be formed by a single a tool having a series of extended spaced blade elements for forming the perforations which extend through the paperboard material along with recessed blade elements interspersed therebetween in alignment with the extended blade elements but which do not pass completely through the paperboard material. Such a cutting tool is difficult and expensive to manufacturer. Moreover, such a cutting tool is readily damaged during use and requires frequent service.
In an effort to overcome such shortcomings, a cutting tool of the type set forth in U.S. Pat. No. 3,255,948 was designed to form a line of weakness in the paperboard material which includes a substantially continuous line which is severed part way through the paperboard material with adjacent perforations being formed therein as well. In doing so, the entire outline of the access opening is severed which prevents the tears between perforations from propagating outside the confines of the access opening; however, such a continuous cut line may weaken the paperboard material about an entire periphery of the access opening which may result in an inadvertent opening of the carton.
Formation of the line of weakness set forth in U.S. Pat. No. 3,019,944 includes inner and outer partial cut scores which are formed in a substantially continuous manner in both the inner surface and outer surface of the top panel of a carton for dispensing flexible sheets. The inner and outer line of weaknesses are spaced from one another a short distance such that when a force is applied to the access opening, ply separation of the paperboard material occurs between the two partial cut scores, thus permitting the removal of the access opening. While this substantially eliminates the possibility of defacing the remaining portion of the top panel, when the access panel is removed, the ply separation, which occurs between the inner and outer partial cut scores, is exposed when the access panel is removed, thus leaving a non-decorative portion of the top panel exposed, which significantly degrades the aesthetics of the carton.
Clearly, there is a need for a carton having an access opening defined by a line of weakness which is both aesthetically pleasing when the carton is in the unopened condition and which provides for a reliable separation of the access opening from the remainder of the carton without defacing the remaining portions of the carton or leaving portions of the inner-ply of the carton exposed. Further, there is clearly a need for a line of weakness which permits the line of weakness to undergo changes in direction without resulting in the defacing of the remaining portions of the carton.